1. Field of the Invention
This invention relates to an ophthalmological measurement method and apparatus, and more particularly to an ophthalmological measurement method and apparatus in which the eye fundus is illuminated by a laser beam having a predetermined diameter and the motion of a laser speckle pattern formed by laser light scattered and reflected from the eye fundus is detected at an observation point as fluctuations in the speckle light intensity to produce a speckle signal which is evaluated
2. Description of the Prior Art
Various conventional methods are used for ophthalmological measurement comprising illuminating the eye fundus with a laser beam, detecting the light scattered by the eye fundus and analyzing and evaluating this light. There are, for example, laser Doppler methods for measuring blood flow in retinal and other tissue described in "Investigative Ophthalmology," vol. 11 No. 11, page 936 (November 1972) and "Science," vol.186 (November 1974) page 830, and in Japanese Unexamined Patent Publication Nos. 55-75668, 55-75669, 55-75670, 52-142885 (corresponding to GB 13132/76 and U.S. Pat. No. 4,166,695), 56-125033 (corresponding to GB 79/37799), 58-118730 (corresponding to U.S. Pat No. 4,402,601) and U.S. Pat. No. 4,142,796. However, these laser Doppler methods involve the use of a high precision optical system, are complicated to use and provide results which lack repeatability and reliability, all of which make practical application difficult.
It is, on the other hand, known that when a laser beam strikes an object which causes diffusion or scattering of the beam, the light scattering from the object gives rise to a speckle pattern caused by interference between reflected rays of the coherent light. The laser speckle method utilizes this to evaluate the state of tissues in the eye fundus. There are, for example, the methods described in Japanese Unexamined Patent Publication Nos. 62-275431 (U.S. Pat. No. 4,734,107 and EPC 234869), 63-238843 (EPC 284248) and 63-242220 (EPC 285314).
These publications describe the use of a detecting aperture to extract time-base fluctuations in the intensity of speckles formed at an optical Fourier Transform plane with respect to the eye fundus, or at the Fraunofer refraction plane, or at an image plane (or a magnified image plane) that is conjugate with respect to the eye fundus, and the blood flow state is determined by an evaluation of the speckle signal thus obtained.
A major obstacle to the clinical application of the above systems has been their susceptibility to the effects of movements, such as movement of the subject's eye, vibration and the like. This frequently causes unwanted movement of speckle patterns on the detection plane, thus throwing the detecting aperture and laser beam out of alignment during measurement. One way to overcome this is described in the laser-Doppler method of Japanese Patent Publication No. 56-125033. This involves the mechanical scanning of the eye fundus image on the detection plane and using differences between the light reflectance of the walls of a blood vessel and that of other areas of tissue to distinguish blood vessels, and correcting for positional deviation. A drawback of this method is that it requires a mechanism for the mechanical scanning of the eye fundus image, which makes the apparatus too large and complex to be practical.
Another method, described in Applied Optics, Vol. 27, No. 6, page 1113 (Mar. 15, 1988) and in Japanese Patent Publication No. 63-288133 (U.S. Pat No. 014994), shows the feasibility of an image scanning arrangement which allows blood vessels to be distinguished and tracked automatically. However, the method is based on the wavelength dependency of reflected light and relies for its implementation on a plurality of laser beams of different wavelengths which are projected in sequence. Again, this makes the apparatus complex, impractical and costly. A further drawback is that when corneal reflection is used to detect eye movement, the detection precision is not high enough for the purposes of correcting for movement by the blood vessel.
Conventional tracking methods involving the detection of eye movement include one in which the corneal surface is illuminated by a laser beam and movement of the reflected light is used to detect and track such eye movement, while another method uses differences between two images of the eye fundus obtained by a TV camera or other such imaging means.
However, such methods involve detection of eye surface movement and are only able to provide a low level of intraocular tracking precision. Moreover, eye fundus images obtained via a TV camera usually suffer from a poor S/N ratio owing to the amount of light being insufficient for the task, and the apparatus required to detect movement based on differences between two images is large and complex.
On the other hand, the speckle pattern moves as the scattering object moves, so that it is proposed to detect its movement as a fluctuation in the light intensity at the observation point to obtain the difference of the traveling speed of the object depending on the signal intensity.
To discriminate the blood vessel parts and measure the diameter of the blood vessel, there has been proposed a method in which the eye fundus is photographed using a fundus camera to measure the diameter of the blood vessel on the basis of the photographed eye fundus or a method in which a television camera is used to take a picture of the eye fundus and the eye fundus image is subjected to an image processing (for example, image sampling, A/D converting, sharpening, masking, filtering) to determine the diameter of the blood vessel.
Such conventional methods need a long time to obtain measurement results because the eye fundus must be photographed, thus making it impossible to measure the diameter of the blood vessel on real time. On the other hand, the eye fundus image taken by the television camera is usually underexposed with a poor S/N ratio. This necessitates a complicated image processing and results in a bulky and expensive apparatus.